Superabsorbent polymers (SAP) are synthetic cross-linked polymeric materials that are capable of absorbing many times their own weight in water and other liquids. Commercially, the materials are used as additives to increase the absorbency of such products as diapers, sanitary napkins, surgical dressings, disposable dust cloths, and the like (hereinafter "absorbent products").
The great commercial significance of these polymers is evidenced by the fact that over 5,000 patents have issued worldwide since 1966 directed to superabsorbent polymers and products. The majority of these have been for end use applications. Much of the current research and development continues to be to directed end use applications, including immobilizing the SAP in the absorbent product. See B. J. Obenski, "SUPERABSORBENT PATENTS, Much More than Just Diapers", Nonwovens Industry, pp 24-28, November 1987.
Because SAPs are significantly cross-linked, it is virtually impossible to put them into solution. Accordingly, SAPs are most commonly used as powders or granules. The use of SAPs in these physical forms presents product design problems as well as health risks. For example, the powdered material has a natural tendency to bunch up or agglomerate within the supporting matrix of the absorbent product. This results in uneven absorptive capacity in the product. Similarly, the fine particulates have a tendency to "dust-off" the supporting matrix resulting in loss of the SAP material altogether.
Powdered SAPs also pose health risks both to end users and those involved in the manufacturing process. The finely powdered SAP can become airborne where it can be inhaled by workers or end users. Once inhaled, the SAP absorbs liquid within the respiratory passages swelling to many times its original size. This can result in blocked air passages and potentially traumatic health complications.
A variety of methods have been suggested to resolve this problem. A conventional approach has been to simply disperse the powdered SAP material in a solid matrix material (e.g. wood pulp, cotton batting, etc.) and fix it in place mechanically as by embossing. That solution fails to completely eliminate bunching and adds costly processing steps.
An alternative to that approach, described in EP Patent Application 255,654, suggests the fabrication of dry formed sheets incorporating cellulose fibers and SAPs. The two materials are suspended in an air stream, fed to a head for dry-forming sheets of paper, laid down on a web, and bound by calendaring and embossing.
To eliminate added processing steps, U.S. Pat. No. 4,826,880 suggests forming hydrates of the SAPs. Such hydrates have reduced tendency to dust off a product and can be used in routine coating processes to coat conventional substrates such as cloth, nonwovens of various fibers, and vinyl films. These hydrates have reduced absorptive properties.
Other approaches effectively glue the particulate SAP material to a fibrous material, which is then mechanically immobilized in the substrate. PCT application WO 90/11811 discloses bicomponent fiber products in which fibers are coated with a liquid binder material. While the binder material is still wet, the particulate SAP is applied resulting in a comprehensive and uniform coating of the matrix fibers. The fibers are then fixed in a fabric or similar substrate by embossing or some such manner.
Still other approaches seek to affix the particulate SAP material to a matrix chemically. European Patent Application 402,650 discloses an absorbent mixture comprising both SAP particles and two phase particles made up of SAP bound to fiber pieces. The absorbent mixture is formed by mixing the monomeric SAP material in a liquid carrier, which is then dispersed within the fiber material, and polymerized and cross-linked "in-situ." Some of the SAP material becomes bound to the fibers. The resulting composite body is ground up to form the absorbent mixture. The absorbent mixture is then sandwiched between thin, nonwoven fibrous layers to form an absorbent article. The absorbent mixture tends to migrate throughout the absorbent article on the basis of particle size.
SAP materials have also been blended with thermoplastic materials for melt extrusion. For example, European Patent Application 425,269 discloses melt-spinnable fibers from thermoplastic materials containing SAP. Among the materials contemplated (but not shown to be suitable) for these fibers is cellulose acetate.
The principal drawback of such melt-spinning processes is that commercially significant superabsorbent polymer materials decompose when heated to temperatures above about 190.degree. C. With decomposition, the SAP's absorbance capacity for aqueous liquids falls off precipitously and it becomes useless.
Cellulose acetate, even when combined with a plasticizer, must be heated to temperatures in excess of 230.degree. to be processed by melt extrusion. Without plasticizer, higher temperatures are required. A number of other commercially significant materials also require temperatures in excess of about 200.degree. C. to be processed by melt extrusion. Thus, it is not feasible to melt extrude SAPs with cellulose acetate or other high melting thermoplastics since the required temperatures are above the decomposition temperature of the superabsorbent polymers.
Additionally, because the superabsorbent polymer decomposes it can not be melted. Instead, when melt extruding SAP-containing materials, the SAP is mixed into the melt where it remains in solid, particulate form. There is an inherent limit to the amount of SAP that can be extruded in that fashion. EP 425,269 teaches that the upper limit of SAP in melt extrusion is 30% by weight. Beyond that point desired qualities of the product are lost.
There is a need for a safe, effective and economical method of immobilizing substantial quantities of superabsorbent polymers in a matrix material. Such a method would not result in thermal decomposition of the SAP and thus diminishment of its absorptive capacity. Further, the method would provide safe and effective absorptive products.